1. Field of the Invention
The present invention relates to a clamping device for machine tools in which a movable carriage, provided linearly movable on a fixed bed, is positioned and fixed.
2. Discussion of the Related Art
In a lathe, for example, that has a headstock secured onto a fixed bed and a movable tailstock, a workpiece is held by a spindle of the tailstock and a chuck of the headstock. Thus, the machining process is carried out by rotating the workpiece. In this case, to prevent undesirable movement of the workpiece, the tailstock must be securely positioned and fixed to the fixed bed with a clamping device.
A conventional clamping device is shown in FIG. 1. In this clamping device, a tailstock 50 is set on a fixed bed 51 with their sliding surfaces 51a, 50a in sliding contact with each other. A piston member 53b is disposed in a hydraulic chamber 53a formed in a pinching plate 53 of the tailstock 50. The piston member 53b is elevated with oil pressure. A clamping force F is supplied to the hydraulic chamber 53a so that a ceiling portion 51b of the fixed bed 51 is pinched between the piston member 53b and the tailstock 50.
Japanese Utility Model Laid-Open Publication No. 2570872 discloses a clamping device in which a protrusion formed at the undersurface of a movable table is pinched by a pair of piezoelectric elements provided on a fixed base.
A structure could be envisioned in which the tailstock 50 is movably supported by linear-motion bearings 55, as shown in FIG. 2, instead of the structure in which the tailstock 50 slides on the fixed bed 51. These linear-motion bearings 55 are generally structured, so that rails 56 fixed to the fixed bed 51 and blocks 57 fixed to the tailstock 50 are engaged with each other with rollers (not shown) interposed in between. In this way, frictional resistance can be reduced.
However, if this type of linear-motion bearing is used in the aforementioned conventional clamping device, there is a possibility that the rails 56 and the blocks 57 may suffer impressions. This occurs as a result of the rollers causing a reaction force f equivalent to the clamping force F acting directly on the linear-motion bearings 55. In addition, when using linear-motion bearings, the clamping force is inferior in proportion to a decrement of friction coefficient, which causes further problems.
In the structure of the aforementioned publication, the table is pinched between piezoelectric elements provided on the base, with a longer travel distance of the table. Therefore it is necessary to set multiple piezoelectric elements in a number corresponding to the travel distance, which disadvantageously increases the number of component parts.
An object of the invention is to provide a clamping device for machine tools capable of preventing the occurrence of impressions, as well as the reduction in clamping force when linear-motion bearings are adopted.
A further object of the invention is to provide a clamping device that avoids any increase in the number of parts.
In one aspect of the invention, there is provided a clamping device for machine tools comprising a movable carriage movably provided on a top face of a fixed bed so that the movable carriage can be positioned and fixed to the fixed bed. The clamping device also comprises:
a guide disposed between the movable carriage and the fixed bed and extending in a direction in which the movable carriage travels so that the movable carriage is supported by said guide;
a recess portion formed in the top face of the fixed bed and extending in the travel direction; and
a cylinder mechanism disposed at an undersurface of the movable carriage, the cylinder mechanism having at least two pistons provided in opposite directions so as to be opposed to left and right inner walls of said recess portion, respectively, so that said inner walls are pressed by the pistons, respectively when said pistons are extended.
In a second aspect of the invention, there is provided a clamping device for machine tools further comprising:
a movable rack fixed to one of the pistons opposed to one of the left and right inner walls, and
a fixed rack fixed to one of said inner walls, wherein the fixed rack is opposed to and engaged with the movable rack.
In a third aspect of the invention, there is provided a clamping device for machine tools comprising a movable carriage movably provided on a top face of a fixed bed so that the movable carriage can be positioned and fixed to the fixed bed. The clamping device also comprises a guide disposed between the movable carriage and the fixed bed and extending in a direction in which the movable carriage travels, the movable carriage being supported by the guide. A strip shaped plate is disposed on the fixed bed extending in the travel direction; and a cylinder mechanism is disposed on an undersurface of the movable carriage. The cylinder mechanism has at least two pistons provided so as to be opposed to each other with the strip shaped plate interposed therebetween so that said plate is pinched by the pistons when the pistons are advanced.
As used herein, the term xe2x80x9cguidexe2x80x9d includes, for example, a linear-motion bearing in which blocks are slidably engaged with rails with rollers or balls interposed in between. Suitable guides also include, for example, a slide guide allowing sliding contact between the movable carriage and the fixed bed at their sliding surfaces.
According to the clamping device in a first aspect of the invention, since a pair of pistons are provided at the undersurface of the movable carriage, and the movable carriage is positioned and fixed by the pistons exerting pressing force on the left and right inner walls within the recess portion of the fixed bed, the clamping force acts only on the fixed bed and never acts on the guide surface. Therefore, for example, when linear-motion bearings are used, the occurrence of impressions as well as a lowering of the clamping force can be prevented.
In addition, since the cylinder mechanism is provided on the movable carriage, it is only necessary to provide a single or small number of cylinder mechanisms regardless of the length of the travel distance of the movable carriage. Thus, the invention avoids the problem of having to increase the number of piezoelectric elements with an increase in length of the travel distance. This problem arises when piezoelectric elements of a conventional clamping device are disposed on the fixed bed side. In contrast, the number of cylinder mechanisms can be minimized, and the cost can be further reduced when using the present invention.
In a second aspect of the invention, since the movable rack is fixed to one of the pistons opposed to one of the left and right inner walls, and the fixed rack with which the movable rack is engaged is fixed to the fixed bed, the clamping force of the movable carriage can be enhanced while utilizing a simple structure.
In a third aspect of the invention, since the strip shaped plate provided on the fixed bed is pinched by a pair of pistons provided on the undersurface of the movable carriage, the clamping force never acts on the guide surface. This prevents the problems of impressions and a lowering of the clamping force. Moreover, only a small number of cylinder mechanisms are required regardless of the length of the travel distance.